Large Volume Holographic Imaging Systems and Associated Methods

ABSTRACT

A method for large volume holographic imaging is provided that may include determining projection operators within sub-volumes of a decomposed target volume, and determining a point aggregation operator for each sub-volume based on the projection operators. The method may further include receiving holographic field measurement data set captured for the target volume via the sensor array, generating a sub-volume interest value for each sub-volume by applying the holographic field measurement data set to each point aggregation operator, determining a sub-volume with a highest sub-volume interest value, and determining respective lower-tier sub-volume interest values for lower-tier sub-volumes of the sub-volume with the highest sub-volume interest value. The lower-tier sub-volumes may be defined by decomposing the sub-volume with the highest sub-volume interest value. Additionally, the method may include generating an image of the target volume based on the lower-tier sub-volume interest values.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/829,150 filed on Apr. 4, 2019, the entire contents of which arehereby incorporated herein by reference, and this application claims thebenefit of U.S. Provisional Application No. 62/829,710 filed on Apr. 5,2019, the entire contents of which are hereby incorporated herein byreference.

TECHNICAL FIELD

Exemplary embodiments generally relate to imaging technologies, and morespecifically relate to holographic imaging technologies.

BACKGROUND

Imaging systems have become increasingly useful, particularly in thecontext of security screening. Such systems are often able to penetrateexterior barriers (e.g., exterior of a suit case or clothing) to revealinformation about the internal features or objects that are visiblyconcealed.

In some instances, imaging systems may leverage, for example, radiowaves that reflect off metals and other materials and can therefore beused for imaging purposes. However, such conventional systems are usefulin relatively small volume environments and often cannot be leveragedfor use, for example, to image or scan a relatively large volumes, suchas an entryway of a stadium, that may have large crowds of passingindividuals. Further, many conventional systems that leverage radiofrequency imaging or scanning, even of small volumes, require complex,high computing power processing that can be expensive to construct andto operate. As such, as need exists to develop more efficient andlower-cost imaging systems that include capabilities to scan or imagerelatively large volumes, for example, for security screening and otherpurposes.

BRIEF SUMMARY OF SOME EXAMPLES

An example method for large volume holographic imaging is provided inaccordance with some example embodiments. In this regard, the method maycomprise decomposing, by processing circuitry, a target volume into aplurality of sub-volumes, and determining, by the processing circuitry,projection operators for points within each sub-volume based on anarchitecture of a sensor array and a spatial relationship of the sensorarray to the target volume. Further, the method may comprisedetermining, by the processing circuitry, a point aggregation operatorfor each sub-volume. In this regard, the point aggregation operator maybe an aggregation of the projection operators associated with arespective sub-volume. The method may also comprise receiving, by theprocessing circuitry, holographic field measurement data captured forthe target volume via the sensor array, generating, by the processingcircuitry, a sub-volume interest value for each sub-volume by applyingthe holographic field measurement data to each point aggregationoperator, determining, by the processing circuitry, a sub-volume with ahighest sub-volume interest value, and determining, by the processingcircuitry, respective lower-tier sub-volume interest values forlower-tier sub-volumes of the sub-volume with the highest sub-volumeinterest value. In this regard, the lower-tier sub-volumes may bedefined by decomposing the sub-volume with the highest sub-volumeinterest value. Further, the method may comprise generating, by theprocessing circuitry, an image of the target volume based on thelower-tier sub-volume interest values.

Additionally, an example apparatus for large volume holographic imagingis provided. In this regard, the apparatus may comprise processingcircuitry configured to decompose a target volume into a plurality ofsub-volumes, determine projection operators for points within eachsub-volume based on an architecture of a sensor array and a spatialrelationship of the sensor array to the target volume, and determine apoint aggregation operator for each sub-volume. In this regard, eachpoint aggregation operator may be an aggregation of the projectionoperators associated with a respective sub-volume. Additionally, theprocessing circuitry may also be configured to receive holographic fieldmeasurement data captured for the target volume via the sensor array,generate a sub-volume interest value for each sub-volume by applying theholographic field measurement data to each point aggregation operator,determine a sub-volume with a highest sub-volume interest value, anddetermine respective lower-tier sub-volume interest values forlower-tier sub-volumes of the sub-volume with the highest sub-volumeinterest value. In this regard, the lower-tier sub-volumes may bedefined by decomposing the sub-volume with the highest sub-volumeinterest value. The processing circuitry may also be configured togenerate an image of the target volume based on the lower-tiersub-volume interest values.

According to some example embodiments, a holography imaging system isprovided that comprises a holography sensor and processing circuitry.The holography sensor may comprise a sensor array, and the holographysensor may be configured to capture holographic field measurement dataof a target volume. The processing circuitry may configured to decomposea target volume into a plurality of sub-volumes, determine projectionoperators for points within each sub-volume based on an architecture ofa sensor array and a spatial relationship of the sensor array to thetarget volume, and determine a point aggregation operator for eachsub-volume. In this regard, each point aggregation operator may be anaggregation of the projection operators associated with a respectivesub-volume. Additionally, the processing circuitry may also beconfigured to receive holographic field measurement data captured forthe target volume via the sensor array, generate a sub-volume interestvalue for each sub-volume by applying the holographic field measurementdata to each point aggregation operator, determine a sub-volume with ahighest sub-volume interest value, and determine respective lower-tiersub-volume interest values for lower-tier sub-volumes of the sub-volumewith the highest sub-volume interest value. In this regard, thelower-tier sub-volumes may be defined by decomposing the sub-volume withthe highest sub-volume interest value. The processing circuitry may alsobe configured to generate an image of the target volume based on thelower-tier sub-volume interest values.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 illustrates an example holography sensor system according to someexample embodiments;

FIG. 2 illustrates an example backscatter array according to someexample embodiments;

FIG. 3 illustrates an example of the holography sensor systemimplemented in the context of a personnel ingress or egress for a venueaccording to some example embodiments

FIG. 4A illustrates a visual image of a duffle bag concealing a pressurecooker;

FIG. 4B illustrates a holographic image of the duffle bag concealing thepressure cooker of FIG. 4A according to some example embodiments;

FIG. 4C illustrates a holographic image of an individual concealing abomb vest according to some example embodiments;

FIG. 5 illustrates a block diagram of a holographic sensor systemaccording to some example embodiments;

FIG. 6 illustrates a process flow for generating and rendering acombined image according to some example embodiments;

FIG. 7 illustrates a flowchart of an example method for generating aholographic field measurement data set and determining image featuresaccording to some example embodiments;

FIG. 8 illustrates a decomposed visualization of a target volumeaccording to some example embodiments;

FIG. 9 illustrates a tree structure that is representative of an examplescenario of a prioritization process according to some exampleembodiments;

FIG. 10A illustrates an example a target area according to some exampleembodiments;

FIG. 10B illustrates an example back projection operator structure thatcorresponds to the target area of FIG. 10A according to some exampleembodiments;

FIG. 10C illustrates an example decomposed sub-areas of the target areaof FIG. 10A after a prioritization process according to some exampleembodiments;

FIG. 10D illustrates the example back projection operator structure ofFIG. 10B indicating the back projection operators that map to thesub-areas of interest of FIG. 10C according to some example embodiments;

FIG. 10E illustrates a tree structure of analyzed sub-areas of FIG. 10Caccording to some example embodiments; and

FIG. 11 illustrates a flowchart of an example method for large volumeholographic imaging according to some example embodiments.

DETAILED DESCRIPTION

Some example embodiments now will be described more fully hereinafterwith reference to the accompanying drawings, in which some, but not allexample embodiments are shown. Indeed, the examples described andpictured herein should not be construed as being limiting as to thescope, applicability or configuration of the present disclosure. Rather,these example embodiments are provided so that this disclosure willsatisfy applicable legal requirements. Like reference numerals refer tolike elements throughout.

As used herein, the terms “component,” “module,” and the like areintended to include a computer-related entity, such as but not limitedto hardware, firmware, or a combination of hardware and software. Forexample, a component or module may be, but is not limited to being, aprocess running on a processor, a processor, an object, an executable, athread of execution, and/or a computer. By way of example, both anapplication running on a computing device and/or the computing devicecan be a component or module. One or more components or modules canreside within a process and/or thread of execution and acomponent/module may be localized on one computer and/or distributedbetween two or more computers. In addition, these components can executefrom various computer readable media having various data structuresstored thereon. The components may communicate by way of local and/orremote processes such as in accordance with a signal having one or moredata packets, such as data from one component/module interacting withanother component/module in a local system, distributed system, and/oracross a network such as the Internet with other systems by way of thesignal. Each respective component/module may perform one or morefunctions that will be described in greater detail herein. However, itshould be appreciated that although this example is described in termsof separate modules corresponding to various functions performed, someexamples may not necessarily utilize modular architectures foremployment of the respective different functions. Thus, for example,code may be shared between different modules, or the processingcircuitry itself may be configured to perform all of the functionsdescribed as being associated with the components/modules describedherein. Furthermore, in the context of this disclosure, the term“module” should not be understood as a nonce word to identify anygeneric means for performing functionalities of the respective modules.Instead, the term “module” should be understood to be a modularcomponent that is specifically configured in, or can be operably coupledto, the processing circuitry to modify the behavior and/or capability ofthe processing circuitry based on the hardware and/or software that isadded to or otherwise operably coupled to the processing circuitry toconfigure the processing circuitry accordingly.

According to various example embodiments, systems, apparatuses, andmethods are provided to holographically image a large space or volume(i.e., a target volume), such as an ingress or egress that individualspass through, using a system comprising a holography sensor. The targetvolume may be, for example, many square meters in size. According tosome example embodiments, the holography sensor may include atransmitter, or illuminator, that illuminates the objects within thetarget volume, Radio waves reflect off of the illuminated objects, andare received at a holographic backscatter array. The holographicbackscatter array may include a plurality of backscatter elements (e.g.,radio frequency resonators). According to some example embodiments, thebackscatter elements may be rapidly switched on (activated) and off(deactivated) by a modulation signal. When activated, the backscatterelements can be excited by the incident radio wave front (e.g., thereflected illumination signal), and will re-radiate or backscatter theradio waves of the illuminator signal as backscatter signals. Whendeactivated, the backscatter elements may output little to no radiowaves. As the backscatter elements rapidly switch between the activatedand deactivated states, the reflected signal provided by a backscatterelement over time is amplitude modulated (AM). In this regard, a signalat frequency Fc that is AM modulated with a signal at frequency Fm, maytherefore be converted to a new signal that contains energy at the sum(Fc+Fm) and differences (Fc−Fm) of the signal and modulationfrequencies. In this way, modulating the backscatter signals of thebackscatter elements, shifts the backscatter signal frequencies from thebackscatter elements away from the illuminating signal frequency,allowing a central radio receiver to isolate the backscatter signalsthat came from actively modulating backscatter elements. According tosome example embodiments, one or more backscatter elements may be activeand modulating at any given time while the other backscatter elementsmay be deactivated. As such, the source of the backscatter signal at anygiven time can be known and the relationship between the position of thesource and data derived from the reflected backscatter signal can beused to generate a holographic image of objects within the targetvolume. In this regard, the backscatter signal from the activebackscatter element may be received by a receiver, converted into data,and aggregated with other data captured in association with otherbackscatter elements of the array. The aggregated data set may bereferred to as a holographic field measurement data set, and may beanalyzed to generate a three-dimensional (3D) representation of thecontents of the target volume.

Because the holographic field measurement data set may include phase andamplitude data, as well as position information for the activebackscatter element, the aggregated data may provide information aboutthe target volume that can be oriented to the physical world in a mannerthat penetrates through visible barriers (e.g., cloth, plastic, etc) toreveal the presence of metal or other dielectric materials in the targetvolume that may otherwise be visibly concealed. As such, because, forexample, radio frequency signals are used, the generated 3D image orrepresentation of the target volume may be used to identify imagefeatures of an object within the target volume that may be visiblyconcealed. Such image features may be used in a variety of ways. Forexample, according to some example embodiments, the image features maybe rendered on a display to be viewed by security personnel.Additionally or alternatively, the image features may be compared with afeature database to identify a match with a suspicious object. Inresponse, to, for example, identifying a match, an alert may begenerated for security screening personnel indicating that an actionshould be taken.

According to some example embodiments, a process of efficiently locatingdata of interest within the holographic field measurement data set mayalso be performed using, for example, an octree decomposition approach.In this regard, the target volume may be decomposed or divided intosub-volumes (e.g., eight top tier sub-volumes) and back projectionoperators determined for points within the sub-volumes may be aggregatedto generate point aggregation operators for each sub-volume. Lower tiersub-volumes may also be defined and point aggregation operators forthose lower-tier sub-volumes may be determined.

Beginning at the top tier sub-volumes, the holographic field measurementdata set may be applied to the point aggregation operators for eachsub-volume to determine a sub-volume interest value for each sub-volume.The sub-volumes may then be further analyzed in order based on themagnitude of the sub-volume interest values. In this regard, thesub-volume with the highest sub-volume interest value may be furtherdecomposed and lower-tier point aggregation operators may be determinedfor the lower-tier sub-volumes of the top-tier sub-volume with thehighest sub-volume interest value. The sub-volumes may continue to bedecomposed until a minimum sub-volume size is reached. Subsequently, thetop-tier sub-volume with the second highest sub-volume interest valuemay be analyzed in the same manner, and so on until all sub-volumes havebeen analyzed or, for example, a timer expires that allows sufficienttime for high interest sub-volumes to be analyzed. Based on thesub-volume interest values, an image may be generated, as furtherdescribed below.

According to some example embodiments, an image may be rendered based onthe holographic field measurement data set on a display to be viewed by,for example, security personnel, and, according to some exampleembodiments, an alert may be generated based on the image for securityscreening personnel indicating that an action (e.g., an investigativeaction) should be taken.

The example embodiments of holography sensor systems, apparatuses, andmethods that are introduced herein may be implemented in the context ofa security screening system for large, crowded venues, such as stadiumsevents, and air and rail transportation hubs. Example embodimentsdescribed herein can offer high-speed scanning or imaging of largecrowds at venues, thereby overcoming the limitation of many conventionalimaging systems that are unsuitable for such large crowd applicationsbecause such system use slow (approximately 5 to 10 second), singleperson scanning techniques and often require the individual to pose incertain position for optimal scanning. Also, such conventional systemsoften use millimeter wavelength technology that can require expensivecomponents. Via some example embodiments described herein, relativelylarge volume spaces can be scanned in three dimensions using radiofrequency signals to support holographic imaging for security screening.

Although example embodiments described herein could be used at variousfrequencies, such as higher frequencies and shorter wavelengthsincluding millimeter wavelengths, some example embodiments may takeadvantage of the benefits of operation at frequencies having, forexample, centimeter wavelengths. Example frequencies that may be usedinclude, but are not limited to, 2.6 GHz signals having about an 11.5centimeter wavelength, 5.8 GHz having about a 5.2 centimeter wavelength,signals between 5 to 6 GHz, or the like. At such wavelengths, theconcealment of a 10 centimeter metal or dielectric object within a crowdof individuals can be imaged using the example holography sensor systemsdescribed herein. Further, for example, 2.6 GHz signals may be used togenerate a holographic image may have a resolution of about 5centimeters, and the signals may easily penetrate clothes and bescattered by both metals and dielectrics.

Also, because components operating at centimeter wavelengths are used inmany other common contexts, (e.g., Wi-Fi, GPS, cellular phones, etc.),the components are more readily available and less expensive allowingsome example embodiments of the holography sensor systems to be bothaffordable and effective. Additionally, certain centimeter wavelengthoperation can be performed in an unlicensed frequency band (e.g., withinthe 5.8 GHz Industrial Scientific and Medical (ISM) band), which canalso reduce cost and increase adoption of the technology. In thisregard, the international ISM radio bands offer a 150 MHz wide licensefree band at 5.8 GHz. Additionally, in the United States, a 600 MHzlicense-free band for radiolocation exists from 5.25 to 5.85 GHz.

Also, because free space propagation loss for electromagnetic wavesincreases proportionally with wavelength, an effective range forholographic imaging at a 5 centimeter wavelength can be 10 times fartherthan that of a conventional 5 millimeter wavelength system. For thisreason, such conventional 5 millimeter wavelength systems cannot providesufficient range for use in large volume contexts, such as for anentryway at a stadium. Some systems that operate at 5 millimeterwavelengths can suffer from higher atmospheric absorption. Whilelaboratory microwave imaging systems can show good performance byoperating over an extended bandwidth, practical deployment may requireoperation within narrowly, allotted bands. As such, according to someexample embodiments as described herein, a holography sensor system mayemploy centimeter wavelength signals, which can reduce system cost,increase the screening range, and increase visual barrier penetration tobetter detect hidden threats in dense, unstructured crowds ofindividuals, in near-real time, while also being non-invasive topersonal privacy.

Further, according to some example embodiments, a prioritization process(or divide and conquer approach) described herein may operate to improvecomputing technology by arriving at the generation of an image (e.g., athree dimensional image) of the target volume at a faster-speed byrequiring less processing. The process, according to some exampleembodiments, may divide the large target volume into a multiresolution,three-dimensional grid that is analyzed from low to high resolution(e.g., from larger sub-volumes to smaller sub-volumes) prioritizingvolumes that include the most data of interest first in the analysis togenerate an image. Accordingly, computing resources may be concentratedon volumes of interest that include, for example, moving people andsuspicious objects. Analyzing the target volume from low resolution tohigh resolution, also allows for consideration of multiple bounce (orreflection) scattering of signals, which can be incorporated into theholographic reconstruction of the target volume, thereby enabling anability to holographically image large, crowded, highly scatteringenvironments.

Having described aspects of some example embodiments in general terms,FIG. 1 illustrates an example holography sensor system 100, according tosome example embodiments. The holography sensor system 100 may comprisean illuminator 110, a receiver 120, an image processor 130, an arraycontroller 140, and a backscatter array 150. The holography sensorsystem 100 may be configured to perform radio frequency holographicimaging of a target volume 170. Various stationary and moving objectsmay be disposed within the target volume 170. In this regard, the targetvolume 170 may be, for example, a personnel corridor or passagewaythrough which individuals gain access to or depart from a venue (e.g., astadium, a retail store, a mass transit station, a school, or the like).The object 171 is representative of an example object that may bepresent within the target volume 170.

The illuminator 110 may be a radio frequency device with a localoscillator that permits the illuminator 110 to output an illuminationsignal 112 at a desired radio frequency (e.g., 2.6 GHz, 5.8 GHz, or thelike) via an antenna. According to some example embodiments, theilluminator 110 may be an interrogator device, such as an RFIDinterrogator device. According to some example embodiments, theillumination signal 112 may have a wavelength between about 1 centimeterto about 15 centimeters. In this regard, the illuminator 110 maycomprise a radio (e.g., a hardware or software defined radio, or ahybrid thereof) and other supporting hardware for generating andcontrolling the output of the illumination signal 112. The illuminator110 may be configured to continuously output the illumination signal 112or the illuminator 110 may be configured to selectively output theillumination signal 112 (e.g., periodically or based on a controlinput). The illuminator 110 may be placed at a location where theillumination signal 112 provides sufficient illumination for interactionwith objects within the target volume 170. As such, according to someexample embodiments, the illumination signal 112 may be required to havea least a threshold power level at any location within the target volume170.

The receiver 120 may be configured to receive backscatter signals fromthe backscatter array 150. In this regard, the receiver 120 may comprisea radio (e.g., a hardware or software defined radio, or a hybridthereof) and other supporting hardware for receiving and controllingreceipt of backscatter signals from the backscatter elements 160 (e.g.,backscatter element 160 a-160 d). According to some example embodiments,the receiver 120 may be a receiver portion of an RFID reader device. Assuch, the receiver 120 may comprise an antenna and radio frequencycomponents configured to convert the received wireless signals into dataindicative of the received wireless signals. In this regard, forexample, the receiver 120 may be configured to receive a wireless signaland generate data indicative of amplitude and phase informationassociated with the received wireless signal. The receiver 120 mayinclude components that attenuate or block the frequency of theillumination signal 112, to permit the receiver 120 to generate databased on amplitude modulated sidebands from the backscatter signals fromthe backscatter array 150. In this regard, the receiver 120 may includea filter that may include or be embodied as a high-pass filter thatattenuates signals at the frequency of the illumination signal 112 toremove the illumination signal 112 from the generated data. According tosome example embodiments, the amplitude and phase information from thebackscatter signals provided by the backscatter array 150 may beextracted from the sidebands.

According to some example embodiments, the illuminator 110 and thereceiver 120 may operate as a single unit. In this regard, according tosome example embodiments, the illuminator 110 and receiver 120 may sharea radio (e.g., which may be a hardware defined, software defined, orhybrid radio) and a local oscillator. According to some exampleembodiments, the radio may be configured for coherent operation betweentransmit and receive channels. In this regard, the illuminator 110 andthe receiver 120 may be combined to form a reader device that isconfigured to both output the illumination signal 112 (which may also bereferred to as an interrogation signal) and receive responsive signalsfrom backscatter elements of the backscatter array 150. The use of acommon local oscillator may allow for coherent measurements of thebackscatter signals, for example, without requiring a loop-backcalibration techniques. Further, with respect to physicalimplementation, the illuminator 110 or the receiver 120, together orseparately, may be installed, for example, in an overhead physicalposition, such as on a ceiling above the target volume 170.

The backscatter array 150 may comprise a plurality of backscatterelements 160 (e.g., backscatter element 160 a-160 d), and may be, forexample, installed along a border or borders of the target volume 170,or even at locations within the target volume 170. In this regard, thebackscatter array 150 may be installed into (or onto) one or more of aceiling, floor, wall, structural column, or the like. As such, the sizeof the backscatter array 150 may be, for example, many feet long by manyfeet wide (e.g., 6 feet by 8 feet). The backscatter array 150 need notbe planar and placement of the backscatter elements need not be uniformas long as the physical position of the backscatter elements 160 withinthe space is known and the power level of the illumination signal 112 atthe location of the backscatter element 160 is sufficient (e.g., above apower level threshold). The backscatter elements 160 may be referred toas backscatter signal emitters or backscatter tags. The backscatterelements 160 may be configured to receive the illumination signal 112and provide a responsive backscatter signal for receipt by the receiver120. The backscatter signal may be a function of the illumination signal112, and since the illumination signal 112 may be impacted by objectswithin the target volume 170, the backscatter signal may includeinformation about the content of the target volume 170 based on theeffect on the illumination signal 112 as received by the backscatterelements 160. In other words, information in a backscatter signal, suchas relative amplitude and phase, can be inferred to have been present inthe illumination signal 112 received at the backscatter element 160.

While the example backscatter array 150 of FIG. 1 shows four backscatterelements 160, according to some example embodiments, any number ofbackscatter elements 160 (e.g., hundreds, thousands, millions, etc.) maybe included in a backscatter array 150. Further, the backscatter array150 may be representative of separate backscatter arrays that may beseparately operated in a coordinated fashion. As such, the backscatterarray 150, and thus, the holography sensor system 100 may be scalablefor a variety of applications and different spaces. The backscatterelements 160 may be formed in, for example, a grid pattern across thearray 150. According to some example embodiments, the backscatterelements 160 may be formed or operated as a sparse array and, in someexample embodiments, a multi-static array. For example, according tosome example embodiments, only some of the grid positions (e.g., formedby a pattern or psuedorandomly selected) within the array 150 may bepopulated with a backscatter element 160. Accordingly to some exampleembodiments, each position within the grid may be populated with abackscatter element 160, and the array 150 may be operated, as furtherdescribed below, as a sparse array by activating one or more of thebackscatter elements 160 at any given time.

Backscatter element 160 a may be representative of each of thebackscatter elements 160 b-160 d, which may have a similar construction.In this regard, the backscatter element 160 a may comprise resonantcircuitry 164 a and an antenna 166 a. According to some exampleembodiments, the resonant circuitry 164 a may be configured to receive asignal at a resonant frequency of the circuitry 164 a (e.g., theillumination signal 112) and provide a responsive backscatter signal, tothe antenna 166 a, that is based on the received signal and a modulationsignal that is provided to the resonant circuitry 164 a. According tosome example embodiments, the resonant circuitry 164 a may comprise achip with a memory that is configured to control the operation of thebackscatter element 160 a. According to some example embodiments, thebackscatter element 160 a may not include a local power source, such asa battery and may therefore be a passively operated device powered bythe energy of the illumination signal 112.

The antenna 166 a may be operably coupled to the resonant circuitry 164a and may be formed as, for example, a dipole or a coil antenna.According to some example embodiments, the antenna 166 a may etched orotherwise formed on an inlay substrate in a manner similar to an RFIDtag. The backscatter element 160 a may also comprise a control switch162 a. The control switch 162 a may be configured to transition thebackscatter element 160 a between an activated state or a deactivatedstate. In this regard, in the activated state, the backscatter element160 a may be configured to receive the illumination signal 112 andtransmit a responsive backscatter signal that is based on theillumination signal 112 that has interacted with the target volume and amodulation signal. In the deactivated state, the backscatter element 160a may be inactive such that the backscatter element 160 a is configuredto not provide a responsive backscatter signal in response to theillumination signal 112. According to some example embodiments, thecontrol switch 162 a may be a component of the resonant circuitry 164 athat, for example, may control the operation of the resonant circuitry164 a and therefore the operation of the backscatter element 160 a. Assuch, the state of the control switch 162 a may determine whether thebackscatter element 160 a is in the active state or the inactive state.However, according to some example embodiments, as further describedbelow, when the backscatter element 160 a is in the active state, thebackscatter element 160 a may be subjected to a modulation signal (e.g.,from the array controller 140) that changes the state of the controlswitch 162 a to incorporate, for example, amplitude modulation into thebackscatter signal of the activated backscatter element 160 a. As such,according to some example embodiments, when the backscatter element 160a is in the active state, the control switch 162 a may be controlled totoggle between the switch states (on or off) based on the modulationsignal. When the backscatter element 160 a is deactivated, the controlswitch 162 a may be maintained in a single state (e.g., off) for theduration of the time that the backscatter element 160 is deactivated.According to some example embodiments, the control switch 162 a may beattached to the antenna 166 a and operation of the control switch 162 amay short circuit or open circuit the antenna 166 a for deactivation.According to some example embodiments, the control switch 162 a may be atransistor, for example, within a chip of the resonant circuitry 164 a.

The array controller 140 may be configured to control the operation ofthe backscatter array 150 and, more specifically, the backscatterelements 160 via the respective control switches (e.g., control switch162 a) of the backscatter elements 160. The array controller 140 mayinclude logic for activating or deactivating various ones of thebackscatter elements 160. The array controller 140 may be configured toactivate one or more of the backscatter elements 160. In an examplescenario, backscatter element 160 a may be the single activatedbackscatter element. As such, according to some example embodiments,backscatter elements 160 b-160 d may be deactivated. In this regard, ifother backscatter elements 160 were selected and activated, thosebackscatter elements 160 may be controlled to operate in the same manneras the backscatter element 160 a. The backscatter element 160 a may beactive for a time slot or a set duration of time that is allocated tothe backscatter element 160 a, such that backscatter signals generatedduring the time slot are known to have been generated by the backscatterelement 160 a and its known position relative to the target volume 170and the antenna of the receiver 120. When the time slot ends, the arraycontroller 140 may be configured to select one or more other backscatterelements 160 to activate, e.g., backscatter element 160 b for thesubsequent time slot. During this subsequent time slot, according tosome example embodiments, all other backscatter elements 160 (i.e.,other than the selected backscatter element or elements) may bedeactivated so that it is known that any backscatter signals during thesubsequent time slot originated from the selected backscatter elementsincluding backscatter element 160 b. As such, according to some exampleembodiments, the array controller 140 may operate to activate only oneof the backscatter elements 160 at a given time.

According to some example embodiments, sets (e.g., more than one) ofbackscatter elements 160 may be selected and activated during a giventime slot, while the unselected backscatter elements 160 aredeactivated. With each subsequent time slot, a different set ofbackscatter elements 160 may be selected and activated, while the otherbackscatter elements 160 are deactivated. With each subsequent timeslot, data from different sets of the backscatter elements 160 can begenerated and gathered. Because the activated backscatter elements 160for each time slot is known, the gathered or aggregated data can beconsidered holistically, by the image processor 130 as described below,to be able to, for example, isolate data from a specific backscatterelement 160 for use in back projection or other techniques.

As such, according to some example embodiments, the one or morebackscatter elements 160 that have been selected, for example, by thearray controller 140, may be activated at any given time, with theidentity (e.g., identification value, location, or the like) of theactivated backscatter elements 160 being known. Activation of a selectedbackscatter element 160 may be performed by via the use of a selectionsignal provided by the array controller 140 to selected backscatterelements 160. According to some example embodiments, the arraycontroller 140 may provide the selection signal to the selectedbackscatter elements to activate the selected back scatter elements.According to some example embodiments, the selection signal may includea modulated signal that may be used by the selected backscatter elements160 to control the operation of the respective control switches (e.g.,control switch 162 a). The modulation signal may formed as a sine wave,square wave, or the like and have a frequency of, for example, in kHz orMHz ranges. Additionally, according to some example embodiments, afrequency of the modulation signal may be selected based on thefrequency of the illumination signal 112 to facilitate the generation ofthe backscatter signals as side bands to the illumination signal 112 forinterference avoidance. In this regard, use of such a modulation signalmay operate to modulate the field of backscatter elements 160 causinggeneration of amplitude modulated sidebands to the frequency of theilluminator signal 112. As such, the backscatter signal provided by aselected backscatter element 160 may be amplitude modulated by mixingwith the modulation signal. In this regard, as mentioned above, a signalat frequency Fc that is AM modulated with a signal at frequency Fm, maytherefore be converted to a new signal that contains energy at the sum(Fc+Fm) and differences (Fc−Fm) of the signal and modulationfrequencies. In this way, the backscatter signal is modulated andshifted in frequency away from the illumination signal 112, allowing thereceiver 120 to isolate the backscatter signals that came from theactive backscatter elements 160. Accordingly, the array controller 140may be configured to activate a selected backscatter element byproviding a modulation signal the selected backscatter element for usein generating the backscatter signal

According to some example embodiments, the array controller 140 may beconfigured to provide the selection signal to the backscatter array 150and more specifically the backscatter elements 160 in a variety ofdifferent ways. In this regard, according to some example embodiments,the array controller 140 may be electrically connected (e.g., viawiring) to each of the backscatter elements 160 to provide the selectionsignal to the selected backscatter elements 160. Alternatively,according to some example embodiments, the selection signal may beprovided via a wireless communication. According to some exampleembodiments, the selection signal may be separated into components. Forexample, according to some example embodiments, the selection signal maycomprise a first component that communicates the selection of thebackscatter elements 160 and a second component that indicates themodulation signal to be used in generating the backscatter signal.

Alternatively, according to some example embodiments, the selectionsignal may be provided optically for addressing the backscatter elements160. In this regard, each backscatter elements 160 may comprise arespect optical sensor that is configured to receive an opticalselection signal and, based on the optical selection signal, transitionthe respective backscatter element 160 to an activated state. In thisregard, if no optical selection signal is received (or if a deactivationoptical selection signal is received), then the respective backscatterelement 160 may transition to a deactivated state. As such, the arraycontroller 140 may, for example, comprise a steerable laser or the likefor transmitting the optical selection signal to the optical sensors ofthe backscatter elements 160. In this regard, array controller 140 maybe configured to provide the optical signal in the form of a scanninglight beam (e.g., laser), a spatial light modulation, a light projection(e.g., a liquid crystal display (LCD) projection), or the like.

According to some example embodiments, the array controller 140 maycontrol the selection of the active backscatter elements 160 (or ratherthe next backscatter elements 160 to activate) in a number of differentways. The array controller 140 may comprise, for example, a switchingnetwork (e.g., in the form of a multiplexer board) for addressing eachof the backscatter elements 160 within the backscatter array 150. Inthis regard, for example, the array controller 140 may be configured toactivate the backscatter elements 160, via the selection signal, in arepeated, sequential order (e.g., in a specific sequential pattern, suchas, a serpentine or other pattern) across the array 150. Alternatively,according to some example embodiments, the array controller 140 mayprovide the selection signal to activate backscatter elements 160 in apseudorandom manner. Additionally, the array controller 140 may be incommunication with the image processor 130, according to some exampleembodiments, to permit the array controller 140 to communicate thebackscatter element selection information (e.g., selected backscatterelements for each time slot) to the image processor 130 for use in lateranalysis. According to some example embodiments, the array controller140 may be controlled by the image processor 130 and, for example, maybe a multiplexed switching device that is controlled by the imageprocessor 130. According to some example embodiments, the backscatterelements 160 may be controlled by the array controller 140 in accordancewith the selection signal, which may comprise, as mentioned above, amodulation frequency, which may be in the kHz or MHz range. Accordingly,by activating backscatter elements 160 across the array 150, a radiofrequency wave front of data about the target volume 170 can begathered, as further described herein.

The image processor 130 may be configured to receive backscatter datafrom the receiver 120 and aggregate the data to generate a collection ofdata for the target volume 170. The image processor 130 may comprise,for example, a processing unit to support the operation of the imageprocessor 130 and the processing unit may be, for example, a graphicprocessing unit (GPU) configured to perform the operations of the imageprocessor 130 described herein. The image processor 130 may beconfigured to receive the backscatter data and generate the collectionof data as a holographic field measurement data set. Since, according tosome example embodiments, the image processor 130 may receive thebackscatter element selection information from the array controller 140,the image processor 130 may link the backscatter element 160 to therespective backscatter data based on the timing of receipt of thebackscatter signal associated with the backscatter data. As such, thebackscatter data may be linked to the particular backscatter element 160(and its position) that generated the respective backscatter signal.Additionally, according to some example embodiments, the modulatedsidebands of the backscatter signals received by the receiver 120 fromthe activated backscatter elements 160, and provided to the imageprocessor 130, may include information that can be extracted byinferring a relative amplitude and phase of a representativeun-modulated wave front incident at the backscatter array 150.

As further, described below, the image processor 130 may be configuredto analyze the holographic field measurement data set to determine thecontents of the target volume 170. In this regard, the image processor130 may be configured to generate a representation (e.g., a 3Drepresentation) of the target volume 170 based on the holographic fieldmeasurement data set. Such a representation may be an image based on thedata provided by backscatter signals from the backscatter elements 160.

Using the holographic field measurement data set, according to someexample embodiments, image features within the image may be identifiedwithin the target volume 170. Since the holographic field measurementdata set is based on radio frequency signals (e.g., centimeterwavelength signals), the holographic field measurement data set mayinclude information indicative of the presence of metal and dielectricswithin the target volume 170. Such information may be converted into animage with visual features for presentation, for example, on the display132, to security personnel or automatically analyzed against a databaseof shapes to identify matches with items of interest (e.g., large metalobjects, incendiary devices, guns, knives, or the like). Because thearray controller 140 may select and activate different backscatterelements 160 at a relatively rapid rate (e.g., kHz, MHz, etc.), dataabout the target volume 170 may be captured and analyzed quickly,thereby enabling, for example, video frame rates and near-real timevideo imaging of the target volume 170 and concealed objects within thetarget volume 170 for presentation to a user, for example, on thedisplay 132.

According to some example embodiments, any technique for converting theholographic field measurement data set into an image may be used. Forexample, various back projection techniques may be used such as aHuygens and Fresnel type back-projection imaging algorithms forholographic imaging as provided in Yang, Yunqiang, “Development of aReal-time Ultra-wideband See Through Wall Imaging Radar System.” PhDdiss., University of Tennessee, 2008; D. M. Sheen, D. L. McMakin, T. E.Hall, “Three-dimensional millimeter-wave imaging for concealed weapondetection”, IEEE Trans. Microw. Theory Tech., vol. 49, no. 9, pp.1581-1592, September 2001; and S. S. Ahmed, A. Genghammer, A. Schiessl,L.-P. Schmidt, “Fully electronic E-band personnel imager of 2 M 2aperture based on a multistatic architecture”, IEEE Trans. Microw.Theory Tech., vol. 61, no. 1, pp. 651-657, 2013, each of which areincorporated by reference in their entirety. Additionally, as furtherdescribed herein, the image processor 130 may be configured to processthe holographic field measurement data set by prioritizing portions ofthe target volume 170 that are likely to have interesting data (e.g.,potential objects of interest) to improve the operation of theprocessing circuitry and reduce the computing resources needed toefficiently generate an image at video frame rates (e.g., 25 frames persecond) in near-real time. Such a prioritization approach may beparticularly useful, or even necessary, in example embodiments where theholographic field measurement data set is based on backscatter signalsoriginating from, for example, millions of backscatter elements 160.

While the illuminator 110, receiver 120, image processor 130, and arraycontroller 140 have been described as separate entities that may bepositioned separately from each other, according to some exampleembodiments, some or all of the illuminator 110, receiver 120, imageprocessor 130, and array controller 140 may be co-located and reliantupon shared components that support the operation of each co-locatedentity. As mentioned above, the illuminator 110 and the receiver 120 maybe formed as a single entity. Similarly, according to some exampleembodiments, the image processor 130 and the array controller 140 may beco-located and supported, for example by common processing devices orprocessing circuitry. According to some example embodiments, theilluminator 110, the receiver 120, the backscatter array 150, and thearray controller 140 may form a holography sensor. Additionally, asmentioned above, the holography sensor system 100 and the holographysensor may be scalable, such that a number of backscatter arrays 150 canbe used in a common system. Such a system may include illuminators 110and receivers 120, for example, for each backscatter array 150, and eachof the receiver 120 may transmit backscatter data, for example, to oneor more image processors 130 for analysis to generate a holographicimage of the target volume 170.

Having described some example components and associated functionalitiesof the holography sensor system 100, a description of the overalloperation of the holography sensor system 100, according to some exampleembodiments, may now be provided. In this regard, the illuminator 110may output the illumination signal 112 in the target volume 170. Theillumination signal 112 may be affected by the presence of the object171 within the target volume 170, and the illumination signal 112 maytherefore be scattered by the object 171 thereby imparting informationabout the target volume 170 into the illumination signal 112 that isultimately received by a backscatter element 160. In this regard,scattered illumination signal 112 may be received by an activebackscatter element 160, i.e., backscatter element 160 a of thebackscatter array 150. The backscatter element 160 a may be activatedvia a selection signal, for example, comprising a modulation signal,provided by the array controller 140, and the array controller 140 mayalso deactivate the other, unselected backscatter elements 160 for atime slot. According to some example embodiments, a duration of the timeslot may be determined based on, for example, processing speeds of thecomponents of the holographic sensor system 100 to permit sufficienttime for the backscatter signals to be transmitted to the receiver 120and stored as backscatter data. The backscatter element 160 a (as wellas any other selected backscatter elements 160) may transmit aresponsive backscatter signal for receipt by the receiver 120. Thereceiver 120 may convert the received backscatter signal intobackscatter data for provision to the image processor 130 and linked tothe backscatter element 160 a based on the time slot.

The array controller 140 may make another selection of one or morebackscatter elements 160, for example, backscatter element 160 c, andbackscatter element 160 c may be activated, while all other backscatterelements 160 may be deactivated for a subsequent time slot. Accordingly,the backscatter element 160 c may transmit a responsive backscattersignal for receipt by the receiver 120. The receiver 120 may convert thereceived backscatter signal into backscatter data for provision to theimage processor 130 and linked to the backscatter element 160 c based onthe subsequent time slot.

The array controller 140 may be continue to select backscatter elements160 and the image processor 130 may continue to collect associatedbackscatter data to generate a holographic field measurement data setfor the target volume 170. In this regard, for example, many thousandsor even millions of backscatter elements 160 may be selected andactivated to output respective backscatter signals. The backscatter datagenerated from the backscatter signals may be collected to form theholographic field measurement data set. Because the backscatter signalsfrom the backscatter elements 160 include information about the targetvolume 170 and the object 171 due to the effect on the illuminationsignal 112 received by the backscatter element 160, an image of thetarget volume 170 can be generated based on the holographic fieldmeasurement data set. In this regard, because the physical placement ofthe of the backscatter elements 160 and the antenna of the receiver 120are known, the information within the backscatter signals (e.g.,amplitude and phase) can be interpreted using, for example, backprojection imaging techniques, in view of these known parameters togenerate the image of the target volume 170. The image may includefeatures that may be associated with objects within the target volume170, and, therefore the image features may be indicative of items ofinterest within the target volume 170, such as the object 171. Forexample, as mentioned above, an image with image features may bepresented to security personnel to trigger an investigation if asuspicious object is identified.

Now referring to FIG. 2, an example backscatter array 200 is shown, inaccordance with some example embodiments. In operation, the backscatterarray 200 may operate similar to the backscatter array 150, however, thebackscatter array 200 may use a column and row connectivity structurefor addressing the backscatter elements.

In this regard, the backscatter array 200 may comprise a plurality ofbackscatter elements 210, including the backscatter elements 210 a and210 x. In the example embodiment shown in FIG. 200, the backscatterarray 200 has 36 backscatter elements 210. Each backscatter element 210may operate in the same manner as described above with respect to thebackscatter elements 160. The backscatter elements 210 may be grouped,for example, onto panels that each have four backscatter elements 210.Due to the column and row connectivity structure, activation of one ormore backscatter elements 210 may be performed by controlling signals ontwo inputs. In this regard, the selection signal provided by the arraycontroller 140 may be provided, for example, as two signals. In thisregard, each of the backscatter elements 210 may comprise logiccircuitry that requires a respective row and column input to be true(e.g., high voltage) for a particular backscatter element 210 to beactive as described above, and the other backscatter elements 210 may beinactive due to the other inputs being false (e.g., low voltage).

The column selection lines C0 to C5 may be inputs of the backscatterarray 200 to select a column, and the row selection lines R0 to R5 maybe inputs to select a row. As such, according to some exampleembodiments, the column selection lines and the row selection lines maybe operably coupled to the array controller 140 to perform selection andactivation of a backscatter element 210. In this regard, to activate thebackscatter element 210 a, the array controller 140 may apply a highvoltage signal (true) on column selection line C0 and row selection lineR0 to activate backscatter element 210 a, and all other column selectionlines and row selection lines may be provided low voltage signal (false)to deactivate all other backscatter elements 210. As another example, toactivate the backscatter element 210 x, the array controller 140 mayapply a high voltage signal (true) on column selection line C2 and rowselection line R4 to activate backscatter element 210 a, and all othercolumn selection lines and row selection lines may be provided lowvoltage signal (false) to deactivate all other backscatter elements 210.

Additionally, according to some example embodiments, the modulationsignal may be applied, for example, as the row or column signal forselecting a backscatter elements 210. In this regard, the modulation ofthe modulation signal may be imparted to the backscatter element 210 viathe selection line (e.g., either the row or the column). A second signalmay be applied to the appropriate row or column to make a selection of abackscatter elements 210, while also providing the modulation signal.

To provide additional context to an example implementation of theholography sensor system 100, FIG. 3 illustrates an example of theholography sensor system 100 implemented in the context of a personnelingress or egress for a venue, according to some example embodiments. Anumber of people 350, including people 350 a to 350 g, are shown asbeing located within the target volume 170. As shown in FIG. 3, theilluminator 110 and the receiver 120 may be co-located at an overheadposition and such that the illumination signal 112 may fully illuminatethe target volume 170. Additionally, an example embodiment of thebackscatter array 150 (possibly similar to the backscatter array 200) isalso shown installed on a wall 310 on a border of the target volume 170.The wall 310 may be a wall of an ingress of egress personnel corridor.The backscatter array 150 is shown as comprising a grid of backscatterelements 210, similar to the backscatter array 200.

As shown in FIG. 3, the illuminator 110 may output the illuminationsignal 112 into the target volume 170 and the illumination signal 112may penetrate through, for example, the people 350 and their clothing,but be scattered by a metallic object 351 being concealed by person 350c. The illustration of FIG. 3 shows a time where the backscatter element160 x has been activated and is providing a backscatter signal 320 tothe receiver 120 based on the scattered illumination signal 112 from themetallic object 351. As described further herein, the backscattersignals from the various activated backscatter elements 160 may beconverted into backscatter data and aggregated to from a holographicfield measurement data set that can be analyzed to generate aholographic image that includes image features of the metallic object351 in near-near real time to, for example, alert security personnel toinvestigate the person 350 c and the objects in person 350 c'spossession.

In this regard, FIGS. 4A to 4C illustrate aspects relating to therendering of a suspicious object on a display for viewing by, forexample, a security personnel, in according to some example embodiments.For example, the renderings of FIGS. 4B and 4C may be based on a systemutilizing centimeter wavelength signals in an example holography sensorsystem 100. In this regard, FIG. 4A illustrates a visual image of aduffle bag 400 with a metallic pressure cooker 410 concealed therein.The metallic pressure cooker 410 may be about a third of a cubic meterin size. The duffle bag 400 may be formed of a woven cloth material thatwould visibly conceal the contents of the duffle bag 400, including themetal pressure cooker 410. However, the use of radio frequencyholographic imaging may penetrate through the woven cloth material ofthe duffle bag 400 and reveal the image features of the pressure cooker410 disposed therein.

FIG. 4B illustrates an example rendering of a holographic image 420 ofthe duffle bag 400 with the metallic pressure cooker 410 disposed in theduffle bag 400. Such holographic image 420 may be rendered on, forexample, a display, such as display 132 of the holography sensor system100. As shown in FIG. 4B, the portion 425 of the holographic image 420associated with the duffle bag 400 is rendered in black because the RFsignals (i.e., the illumination signal 112) are able to penetratethrough the woven cloth material of the duffle bag 400. However, the RFsignals (i.e., the illumination signal 112) are scattered by themetallic pressure cooker 410 and therefore the rendering of theholographic image 420 includes a brighter area 430 that is associatedwith the metallic pressure cooker 410 within the duffle bag 400.

Now referring to FIG. 4C, another holographic image 440 is shown thatmay be generated in accordance with some example embodiments. Theholographic image 440 is of an individual with a bomb vest positioned inthe chest region. Again, because the RF signals (i.e., the illuminationsignal 112) may be of a wavelength that penetrates through a body, theportion 445 of the holographic image 440 that is rendered in black isassociated with the individual. However, the RF signals (i.e., theillumination signal 112) are scattered by the bomb vest, due to the vestbeing constructed of metallic or other dielectric material, andtherefore the rendering of the holographic image 440 includes a brighterarea 450 that is associated with the bomb vest.

Now referring to FIG. 5, a block diagram of another example embodimentof a holography sensor system 500 is provided. The holography sensorsystem 500 comprises elements of the holography sensor system 100 thatare controlled or embodied by processing circuitry 505 to perform thefunctionalities of the components described herein, such as, theilluminator 110, the receiver 120, the image processor 130, and thearray controller 140. The holography sensor system 500 also comprisesthe backscatter array 150 and the display 132. The processing circuitry505 may be distributed amongst some or all of the components orcentralized. Processing circuitry 505 may, in turn, comprise a processor515 and a memory 510.

Further, according to some example embodiments, processing circuitry 505may be in operative communication with or embody, the memory 510 and theprocessor 515. Through configuration and operation of the memory 510 andthe processor 515, the processing circuitry 505 may be configurable toperform various operations as described herein, including the operationsand functionalities described with respect to the illuminator 110, thereceiver 120, the array controller 140, and the image processor 130. Inthis regard, the processing circuitry 505 may be configured to performcomputational processing and memory management, user interface control,and the like to support the implementation of other functionalities. Insome embodiments, the processing circuitry 505 may be embodied as a chipor chip set. In other words, the processing circuitry 505 may compriseone or more physical packages (e.g., chips) including materials,components or wires on a structural assembly (e.g., a baseboard). Theprocessing circuitry 505 may be configured to receive inputs (e.g., viaperipheral components), perform actions based on the inputs, andgenerate outputs (e.g., for provision to peripheral components). In anexample embodiment, the processing circuitry 505 may include one or moreinstances of a processor 515, associated circuitry, and memory 510. Assuch, the processing circuitry 505 may be embodied as a circuit chip(e.g., an integrated circuit chip, such as a field programmable gatearray (FPGA)) configured (e.g., with hardware, software or a combinationof hardware and software) to perform operations described herein.

In an example embodiment, the memory 510 may include one or morenon-transitory memory devices such as, for example, volatile ornon-volatile memory that may be either fixed or removable. The memory510 may be configured to store information, data, applications,instructions or the like for enabling, for example, the functionalitiesdescribed herein. The memory 510 may operate to buffer instructions anddata during operation of the processing circuitry 505 to supporthigher-level functionalities, and may also be configured to storeinstructions for execution by the processing circuitry 505. The memory510 may also store various information including backscatter data andthe holographic field measurement data set. According to some exampleembodiments, various data stored in the memory 510 may be generatedbased on other data and stored or the data may be retrieved via acommunications interface and stored in the memory 510.

As mentioned above, the processing circuitry 505 may be embodied in anumber of different ways. For example, the processing circuitry 505 maybe embodied as various processing means such as one or more processors515 that may be in the form of a microprocessor, graphical processingunit, or other processing element, a coprocessor, a controller orvarious other computing or processing devices including integratedcircuits such as, for example, an ASIC (application specific integratedcircuit), an FPGA, or the like. In an example embodiment, the processingcircuitry 505 may be configured to execute instructions stored in thememory 510 or otherwise accessible to the processing circuitry 505. Assuch, whether configured by hardware or by a combination of hardware andsoftware, the processing circuitry 505 may represent an entity (e.g.,physically embodied in circuitry—in the form of processing circuitry505) capable of performing operations according to example embodimentswhile configured accordingly. Thus, for example, when the processingcircuitry 505 is embodied as an ASIC, FPGA, or the like, the processingcircuitry 505 may be specifically configured hardware for conducting theoperations described herein. Alternatively, as another example, when theprocessing circuitry 505 is embodied as an executor of softwareinstructions, the instructions may specifically configure the processingcircuitry 505 to perform the operations described herein.

As mentioned above, the processing circuitry 505 may be configured toperform or control the performance of the functionalities of theilluminator 110, the receiver 120, the array controller 140, and theimage processor 130, as well as other functionalities described herein.In this regard, the illuminator 110 may be configured to output aillumination signal into a target volume 170. In this regard, afrequency of the illumination signal may be a radio frequency (e.g.,having a centimeter wavelength). The array controller 140 may beoperably coupled to the backscatter array 150 and, more specifically,the plurality of backscatter elements of the backscatter array 150,which may, for example, be positioned along a boundary of the targetvolume. In this regard, the array controller 140 may be configured toselect a backscatter element from the plurality of backscatter elements.Further, the array controller 140 may be configured to activate theselected backscatter element to enable the selected backscatter elementto transmit a backscatter signal in response to receipt of theillumination signal, and deactivate the unselected backscatter elementsof the plurality of backscatter elements. Further, via the processingcircuitry 505, the receiver 120 may be configured to receive thebackscatter signal from the selected backscatter element.

The image processor 130, via the processing circuitry 505, may beconfigured to receive a transmission of the backscatter signal from thereceiver 120 and convert the transmission of the backscatter signal intobackscatter data. Further, the image processor 130 may be configured toaggregate the backscatter data for the selected backscatter element withdata associated with other backscatter elements of the plurality ofbackscatter elements to form a holographic field measurement data set,and determine image features of an object within the target volume basedon the holographic field measurement data set.

Further, according to some example embodiments, the array controller 140may be configured to select the backscatter element based on apseudorandom selection procedure. Further, the array controller 140 maybe configured to activate the selected backscatter element for a firsttime slot and further configured to select a subsequent backscatterelement for activation during a second time slot. Additionally oralternatively, the array controller 140 may be configured to activatethe selected backscatter element via the row and column connectionstructure. Further, the receiver 120 may be configured to convert abackscatter signal into backscatter data indicative of amplitude andphase.

According to some example embodiments, the image processor 130, via theprocessing circuitry 505, may be configured to perform operationsassociated with security screening procedures. In this regard, the imageprocessor 130 may be configured, for example, to use back projection,such as the back projection techniques described herein, to generate animage based on the holographic field measurement data set. The image mayinclude image features indicative of an object (e.g., a metal or otherdielectric object) that is located within the target volume 170. Basedon a size or shape of the object, as provided by the image features, theimage processor 130 may be configured to generate a security screeningalert. Further, the image processor 130 may be configured to render theimage or a portion of the image including the image features of theobject on the display 132 to be viewed, for example, by securitypersonnel. According to some example embodiments, the image processor130 may be configured to track the movement of an object based on theimage features of the image through repeated generation of an image ofthe target volume 170 in response to receipt of updated or newholographic field measurement data sets.

With regard to rendering a holographic image, the image processor 130may be configured to render the holographic image by combining theholographic field measurement data set after conversion into an RFhologram with other imaging data. In this regard, with reference to FIG.6, a process flow for generating an rendering a combined image isprovided that may be performed by the image processor 130.

In this regard, the holographic field measurement data set may beconverted into an RF hologram 610 via, for example, a holographytechnique such as back projection as described herein. Additionally, adepth map 620 of, for example, the target volume 170, may also begenerated using, for example, a depth camera (such as a MicrosoftKinect™) that maps depth via structured light or time of flightmeasurements to generate the depth map 620. At 630, the RF hologram 610may be combined with the depth map 620 via a depth constrainedreconstruction, and, as a result, an RF reconstruction in the form of anRF reflectively map 640 may be formed. In this regard, because both theRF hologram 610 and the depth map 620 include depth or distance-basedinformation, the RF hologram 610 and the depth map 620 may be combinedbased on a correlation between the depth or distance-based information.Additionally, the depth map 620 may introduce visual features into theRF reflectively map 640. The RF reflectivity map 640 may, in turn, becombined with an image of the target volume 170, captured via a camera.To do so, a registration process may be undertaken at 660 to align theRF reflectively map 640 with the image 650. The registration process maybe performed based on the visual features of the RF reflectively map 640or a priori. As a result, a superimposed reflectivity map 670 may begenerated that highlights overlays the threshold-level data from the RFreflectivity map 640 on the image 650, thereby indicating, in theexample shown in FIG. 6, that an item of interest may be present in theindividual's shirt pocket.

Referring now to FIG. 7, a flowchart of an example method 701 forgenerating a holographic field measurement data set and determiningimage features is provided. The example method 701 may comprise, at 700,outputting an illumination signal into a target volume. In this regard,a frequency of the illumination signal may be a radio frequency. Theexample method 701 may further comprise, at 710, selecting a firstbackscatter element from a plurality of backscatter elements in abackscatter array. According to some example embodiments, the selectedbackscatter element may be one of a set or plurality of selectedbackscatter elements. According to some example embodiments, thebackscatter elements may be positioned along a boundary of the targetvolume. Further, at 720, the example method 701 may comprise activatingthe first backscatter element to enable the first backscatter element totransmit a first backscatter signal in response to receipt of theillumination signal, and, at 730, deactivating an unselected backscatterelement of the plurality of backscatter elements. According to someexample embodiments, the unselected backscatter element may be one of aset or plurality of unselected backscatter elements. At 740, the examplemethod 701 may comprise receiving the first backscatter signal from thefirst backscatter element and generating first backscatter data based onthe backscatter signal.

According to some example embodiments, the example method 701 may alsocomprise, at 750, receiving the first backscatter data, and, at 760,aggregating the first backscatter data with other backscatter data toform a holographic field measurement data set. Further, at 770, theexample method 701 may comprise generating an image of the target volumebased on the holographic field measurement data set.

According to some example embodiments, the example method 701 may alsocomprise, prior to generating the image at 770, selecting a secondbackscatter element from the plurality of backscatter elements. In thisregard, the selection of the second backscatter element may include notselecting the first backscatter element. Further in this regard, theexample method 701 may comprise activating the second selectedbackscatter element to enable the second backscatter element to transmita second backscatter signal in response to receipt of the illuminationsignal, and deactivating unselected backscatter elements including thefirst backscatter element. Additionally, the example method 701 maycomprise receiving the second backscatter signal from the secondbackscatter element.

According to some example embodiments, the example method 701 mayfurther comprise receiving a second transmission of the secondbackscatter signal and converting the second transmission of the secondbackscatter signal into second backscatter data. Additionally, theaggregating may comprise aggregating the first backscatter data with thesecond backscatter data to form the holographic field measurement dataset. Further, the example method 701 may comprise generating the imageof the target volume based on the holographic field measurement data setthat includes the aggregation of the first backscatter data with thesecond backscatter data.

Additionally, according to some example embodiments, converting thefirst transmission of the first backscatter signal may compriseconverting the first transmission of the first backscatter signal intothe first backscatter data, where the first backscatter data may beindicative of amplitude and phase of the first backscatter signal.Additionally or alternatively, according to some example embodiments,selecting the first backscatter element may comprise selecting the firstbackscatter element based on a pseudorandom selection procedure.According to some example embodiments, activating a selected backscatterelement (e.g., the first backscatter element or the second backscatterelement) may comprise activating the selected backscatter element (e.g.,the first backscatter element) for a first time slot and selecting asubsequent backscatter element (e.g., a second backscatter element) foractivation during a second time slot. Additionally or alternatively,activating the first backscatter element may include providing amodulation signal to the first backscatter element for use in generatingthe backscatter signal. Additionally or alternatively, activating abackscatter element may comprise activating the backscatter element viaa row and column connection structure. Additionally, according to someexample embodiments, the example method 701 may also comprise generatinga security screening alert based on the image features of the object.

As mentioned above, the holographic field measurement data set mayinclude a vast amount of data that, according to some exampleembodiments, must be analyzed quickly to support video frame rates andnear-real time imaging of the target volume 170. In this regard, forexample, a typical public venue ingress area might be 10 meters by 10meters by 2.5 meters, thus having a relatively large volume. A usefulresolution for a security screening system may be about 12.5 cubiccentimeters, which may be achieved by leveraging data points spaced at2.5 centimeters. To implement such a system may require the use of over32 million half wavelength spaced points (e.g., backscatter elements orother sensor antennas) and associated data. Based on such an examplesystem and the desire for video frame rates, techniques may be employedas described herein, for example, by the image processor 130, to reducethe processing resources required to generate an image from theholographic field measurement data set. After obtaining the holographicfield measurement data set, a prioritization process may be undertakento target the data that is most interesting, according to some exampleembodiments.

According to some example embodiments, a prioritization method andassociated apparatuses configured to perform the method may beimplemented to improve the efficiency of generating an image based aholographic field measurement data set. In this regard, the holographicfield measurement data set may be generated, according to some exampleembodiments, in any fashion using any holographic imaging system. Theholographic sensor system 100 may be one example of a system that may beused to generate a holographic field measurement data set, but othersystems may be used that also leverage an array of sensors, such as anarray of antenna sensors. In addition to prioritizing analysis onportions of the target volume 170 that are likely to be most interestingor valuable first, according to some example embodiments, projectionoperators and aggregations of the projection operators may bepredetermined and organized into a tree structure to facilitateincreased efficiency for generating an image based on the holographicfield measurement data. Using these techniques, the computations thatmay be required to generate an image from the holographic fieldmeasurement data sets that are being continuously captured and providedcan be reduced and simplified, thereby improving the performance ofimage processing and supporting near-real time rendering of video framerates. Additionally, while example embodiments of the prioritizationapproach provided herein are described in the context of data sets basedon radio frequency holography, the approach may also be applied to datasets collected using other types of holography, such as, for example,acoustic holography.

In this regard, based on a desired resolution, a plurality of points inspace may be defined within the target volume 170. For example, thepoints may be organized in a three-dimensional grid and have a defineddensity. For each defined point within the target volume 170, backprojection operators (or projection operators) may be calculated basedon an architecture of a sensor array (e.g., backscatter array 150 or anarray of antennas) used to capture the holographic field measurementdata set and the spatial relationships between each point and theelements of the sensor array. The projection operators may, according tosome example embodiments, be defined based on a retracement or backprojection of the wave from a particular sensor element of a sensorarray with respect to the defined point and a source (e.g., theilluminator 110). Since these projection operators are a function thearchitecture of the sensor array and the spatial relationship betweenthe sensor array and the target volume 170, the projection operators maybe determined in advance of any holographic field measurement data setcapturing by the sensor array. As such, determination of the projectionoperators may be performed as part of a setup operation, and thereforedo not need to be determined at runtime, when holographic fieldmeasurement data set is being captured and provided for analysis.

Additionally, based on a decomposition scheme, the target volume 170 maybe decomposed or divided into sub-volumes, where each sub-volume definesa portion of the target volume 170. In this regard, for example, asshown in FIG. 8, a simplified visualization of the target volume 170 isprovided as a cubic space being decomposed into sub-volumes. Forexample, if an octree decomposition approach is used, according to someexample embodiments, then the target volume 170 may be segmented intoeight sub-volumes 800, namely, sub-volumes 800 a to 800 h. Again, forsimplification the sub-volumes 800 are illustrated as equally sized,cubic volumes, but other sized and shaped decompositions may be useddepending on the approach. Additionally, although not shown, eachsub-volume 800 may be further decomposed into lower-tier sub-volumes. Inthis regard, each sub-volume 800 may be decomposed into eight lower-tiersub-volumes. According to some example embodiments, further levels ofsub-volumes may also be defined until a sub-volume size is reached thatis a minimum desired size for a particular application. Thedecomposition of the sub-volumes in this manner may formed into atree-structure that defines the relationships between the various levelsof defined sub-volumes.

Since each sub-volume (regardless of the level of the sub-volume)defines a volume in space within the target volume 170, each sub-volumemay have points, as described above, that are located within thesub-volumes. As such, the points within a given sub-volume may beassociated with the sub-volume. Additionally, the projection operatorsfor the points within a sub-volume may also be associated with thesub-volume. As such, a group or collection of projection operators maybe associated with each sub-volume, regardless of the level of thesub-volume on the tree structure. For each sub-volume, the group ofprojection operators, as linear operators, may be organized into amatrix (e.g., an M×N matrix, where M is there number of points in thesub-volume and N is the number of sensor elements in the sensor array).

According to some example embodiments, as a preliminary operation (i.e.,prior to receiving regular holographic field measurement data set, thegroup of projection operators for the sub-volumes may be aggregated intoa combined operator for the sub-volume, which may be referred to as thepoint aggregation operator for the sub-volume. In this regard, accordingto some example embodiments, the group of projection operators may besubjected to a linear operation to perform an aggregation to generatethe point aggregation operator. For example, the group of projectionoperators for the sub-volume may be averaged or summed to generate thepoint aggregation operator for the sub-volume. Because a linearoperation may be used to aggregate or combine the projection operatorsfor the sub-volume, the resultant matrix that describes the pointaggregation operator may be reduced to a one dimensional matrix (e.g.,an 1×N matrix). According to some example embodiments, a weighting, forexample in the form of a windowing function, may be applied, based on,for example, a spatial location of the sub-volume. In this regard,sub-volumes that may be located central to the target volume 170 orsub-volumes that are spatially associated with, for example, high foottraffic areas, may be applied a higher weighting over sub-volumes along,for example, a periphery of the target volume 170 were no or little foottraffic occurs. In association with the sub-volumes, the pointaggregation operators may be organized in a tree structure, such as anoctree structure.

The generation of the point aggregation operators for the sub-volumesmay prepare the system for receipt and analysis of the holographic fieldmeasurement data set. However, in some instances, for example, due tothe memory requirements for storing the point aggregation operators forall possible sub-volumes, according to some example embodiments, pointaggregation operators may be determined for only some sub-volumes, suchas those that are located in high interest locations within the targetvolume 170. As such, in situations where the point aggregation operatormay be needed for a given sub-volume, as further described below, thepoint aggregation operator may be determined at runtime when holographicfield measurement data set is be captured and analyzed.

In this regard, when holographic field measurement data set is received,the data can be applied to point aggregation operators for the first ortop-tier sub-volumes. As a linear operation, the results of theapplication of the holographic field measurement data set to the pointaggregation operator for each sub-volume may be a value, such as acomplex number. As such, upon application of the holographic fieldmeasurement data set to each point aggregation operator, a valuereferred to as the sub-volume interest value for each top-tiersub-volumes may be determined. Additionally, because the determinationof the sub-volume interest value may be a linear operation, theprocessing resources utilized to generate the sub-volume interest valuemay be minimal, in particular, relative to performing a conventionalback-projection analysis for the points in the sub-volume.

A magnitude of the sub-volume interest value may be indicative of thequantity of interesting data located within the associated sub-volume ofthe target volume 170. For example, sub-volumes that contain large metalobjects located therein may return a higher magnitude sub-volumeinterest value than a sub-volume that is empty or includes no metal ordielectric objects. Accordingly, the sub-volume with the highestsub-volume interest value may be of the most interest and processing maybe initially focused on this sub-volume.

According to some example embodiments, the sub-volumes may be organizedor pushed onto a stack based on their respective sub-volume interestvalue. In this regard, for a first-in, last-out stack, the sub-volumewith the lowest sub-volume interest value may be pushed onto the stackfirst, followed by the sub-volume with the second lowest sub-volumeinterest value, and so on until the sub-volume with the highestsub-volume interest value is pushed on the stack last. As such, thesub-volumes may be organized for highest sub-volume interest value tolowest sub-volume interest value, so that the sub-volume with thehighest sub-volume interest value is analyzed first and then subsequentanalyses are performed in decreasing order of sub-volume interestvalues. Additionally or alternatively, sub-volumes with sub-volumeinterest values that do not satisfy an elimination threshold may bediscarded for including too little information of interest and nofurther analysis of these discarded sub-volumes may be undertaken.

Regardless of the manner in which the sub-volumes are organized based onthe sub-volume interest values, the sub-volume with the highestsub-volume interest value may be the first sub-volume that is subjectedto further analysis. In this regard, moving to a next lower level ortier on the tree structure for the sub-volume with the highestsub-volume interest value, a number (e.g., eight) leaves may be definedthat are associated with point aggregation operators for the lower-tiersub-volumes that make up the sub-volume with the highest sub-volumeinterest value. As such, similar to the process above, the holographicfield measurement data set may be applied to the lower-tier pointaggregation operators to generate lower-tier sub-volume interest valuesfor the lower-tier sub-volumes. Again, these lower-tier sub-volumes maybe organized based on the lower-tier sub-volume interest values and thelower-tier sub-volume with the highest sub-volume interest value may befurther decomposed and analyzed at the next level in the tree structure.This process may continue until the size of the next level ofsub-volumes is less than a defined minimum size.

Upon completion of the tree-structure analysis of the sub-volume withthe highest sub-volume interest value, the sub-volume with the secondhighest sub-volume interest value may be analyzed to generate sub-volumeinterest values in the same manner. Each of the sub-volumes with thenext highest sub-volume interest values may be subsequently analyzed ina similar manner. The process may continue, for example, via analysis ofa next sub-volume on the stack or until a timer expires. Such a timermay be set based on the timing required to provide, for example, videoframe rates. In this regard, the timer may be set for long enough toassume that any sub-volumes with data of sufficient interest have beensufficiently analyzed when the timer expires.

Additionally, according to some example embodiments, the process ofmoving to a next lower level of the tree structure for analysis ofsmaller sub-volumes may be halted for a sub-volume if the sub-volumeinterest value is higher than a projection threshold value. In thisregard, according to some example embodiments, a sub-volume may bedetermined by the sub-volume interest value to have such as large amountof interesting data that further lower level analysis of the sub-volumemay result in low value processing that may be more resource intensivethan performing back projection imaging for the sub-volume. In suchinstances, performing back projection imaging with respect to thesub-volume having sub-volume interest value that exceeds the projectionthreshold value may, according to some example embodiments, consume lessprocessing resources than determining lower-tier sub-volume interestvalues. As such, the projection threshold may be set at a level wheresuch a condition may exist.

Having generated the sub-volume interest values for a sub-volumes at thelowest levels of the tree structure, an image may be rendered based onthe sub-volume interest values. In this regard, the lowest levelsub-volumes may be used as voxels (three dimensional pixels) within arendering of an image of the target volume 170. For example, a binaryrendering threshold may be applied. In this regard, if the sub-volumeinterest value is greater than a coloring threshold, an object ofinterest may be located in the sub-volume and the rendered voxelassociated with the sub-volume may be colored a first color (e.g., red).If, however, if the sub-volume interest value is less than a coloringthreshold, an object of interest may not be located in the sub-volumeand the voxel associated with the sub-volume may be colored a secondcolor (e.g., white or transparent).

In view of the above, FIG. 9 illustrates an example tree structure 900that is representative of an example scenario for the prioritizationprocess described above. In this regard, the circles are representativeof some of the defined sub-volumes. The circle 905 may be representativeof the entire target volume 170. Further, circles 910 a to 910 h may berepresentative of the top-tier sub-volumes that make up the targetvolume 170 (and may correspond to the sub-volumes defined in FIG. 8,i.e., sub-volumes 800 a to 800 h). The circles 910 a to 910 h have anassociated value shown in each circle that is representative of anexample sub-volume interest value for the respective sub-volume. Thesub-volume interest value may be determined based on the predeterminedpoint aggregation operator for each sub-volume as described above. Inthis regard, it can be seen that circle 910 d has the highest sub-volumeinterest value, i.e., 8, and therefore further analysis of thesub-volume associated with circle 910 d may be initially undertakensince that sub-volume is most likely to have interesting data. As shown,in FIG. 9, a lower-tier or level of sub-volumes for the sub-volumeassociated with circle 910 d are shown. Again, sub-volume interestvalues for these lower-tier sub-volumes can be determined based on therespective point aggregation operators for the lower-tier sub-volumes.In this regard, the sub-volume associated with circle 920 c has thehighest sub-volume interest value. However, in the example scenario ofFIG. 9, a minimum sized sub-volume has been reached (e.g., fiftycentimeters cubed), and therefore no further analysis of the lower-tiersub-volumes 920 a to 920 h are performed. Subsequently, decompositionmay be performed with respect to sub-volume associated with circle 910 fas the top-tier sub-volume with the second highest sub-volume interestvalue. As such, lower-tier sub-volume interest values may be determinedfor the lower-tier sub-volumes of sub-volume represented by circle 910f. The process may continue for each of the top tier sub-volumes inorder from highest to lowest sub-volume interest value until all thelower-tier sub-volume interest values are determined or a time outoccurs. Subsequently, the lower-tier sub-volumes may be rendered in animage based on the lower-tier sub-volume interest values that have beendetermined.

According to some example embodiments, since back projection andgenerating the sub-volume values via averaging may be linear operations,a matrix of projection operators, as described above, may be formed forback projection and generation of the sub-volume interest values. Thedetermination of the sub-volume interest values may be realized as aconvolution in space, or as multiplication or windowing operation on,for example, the sensor array. Convolution in space may becomputationally expensive, but because the projection matrices can bepre-calculated and stored for use at runtime, the effect on video framerates and near-real time operation may not be affected. In this regard,use of the point aggregation operators allows for the imaging operationsto be a series of efficient matrix-vector multiplications. As anotheroption, filtering by windowing with respect to the array may be morecomputationally efficient, in some instances, than convolution but lessthan matrix-vector multiplication. In this regard, windowing andback-projection imaging may be accomplished in near-real time, therebyeliminating the need for large amounts of memory. However, according tosome example embodiments, the calculations for windowing andback-projection may involve relatively processing expensive square-rootsand complex exponentials. Both approaches may depend on the sparsity thesub-volume being imaged. As the sub-volume becomes more densely filled,according to some example embodiments, the tree-structure (e.g., octree)overhead may become burdensome (relative to a bruit force backprojection), and therefore implementation of the projection threshold ateach level of the tree may become useful with respect to minimizing theprocessing overhead. In this regard, if projection threshold isexceeded, as described above, back projection of the full sub-volume maybe performed, even if the minimum sized sub-volume has not been reached.The predetermination of the projection operators and the pointaggregation operators, can therefore improve the efficiency of theprocessing systems to provide near-real time video frame rate outputs,since, for example, the point aggregation operators can be organized ina tree structure stored in advance, locally to the image processor 130for rapid retrieval and use. According to some example embodiments, asmentioned above, memory limitations may, in some instances, causestorage of all point aggregation operators for all sub-volumes of thetarget volume 170 to be impractical. If a point aggregation operator fora sub-volume has not been predetermined then determination of theprojection operators and the point aggregation operators, for thesub-volume that do not have predetermined point aggregation operators,can be determined after the holographic field measurement data set isreceived.

Having described the approach in a general sense, imaging using apre-calculated projection octree can now be described in further detailin the context of an example. In this regard, a three-dimensional imageof an approximate reflection coefficient at time t,

(t), can be calculated by applying a reflection coefficient operator, R,to a vector of measurements Q(t), thereby providing

(t)=RQ(t). The reflection coefficient operator may be constructed usingthe Born approximation with simple Huygens back projection, for example,as provided in D. M. Sheen, D. L. McMakin, T. E. Hall,“Three-dimensional millimeter-wave imaging for concealed weapondetection”, IEEE Trans. Microw. Theory Tech., vol. 49, no. 9, pp.1581-1592, September 2001, which is herein incorporated by reference inits entirety. The images considered in this example may bethree-dimensional images over a regular three-dimensional gridrepresentative of the target volume. The sampling of the grid may bedetermined by the Nyquist rate of the holographic sensor system.

According to some example embodiments, a prioritization process usingthe octree decomposition approach can efficiently determine volumes ofinterest without fully reconstructing the entire target volume asmentioned above. The projection operators used for determining the pointaggregation operators may, according to some example embodiments, be theproduct of a sequence of projections of three-dimensional wavelettransforms and the reflection coefficient operator for a particularsub-volume of the grid. As such, the operators may produce filtered anddown-sampled reflection coefficient images. In this regard, FIGS. 10Aand 10B illustrate two-dimensional representations of a target space andprojection operators in an implementation where the projection operatorsare separately considered. Referring to FIG. 10A, the target area 1000(as opposed to a target volume) is represented in association with itsidentifier D. FIG. 10B shows an example octree decomposition 1010 of thetarget area in association with respective projection operators thatcorrespond to the data set D, with the respective operators being p₁ top₆₄.

In the example scenario, as shown in FIGS. 10C and 10D, the sub-volumes,here sub-areas D_(1,1), D_(3,37), and D_(2,15) of D, have beendetermined to have data of interest as provided by the darkened areas inthe decomposed target area 1020. Accordingly, the mapped projectionoperators shown in the mapped octree decomposition 1030 may be applied,as indicated by the darkened areas that are spatially mapped to the sameareas in the decomposed target area 1020 of FIG. 10C. In this regard,for the sub-area D_(1,1), projection operators p₁ to p₄, p₉ to p₁₂, p₁₇to p₂₀, and p₂₅ to p₂₈ may be utilized. For the sub-area D_(3,27),projection operator p₃₇ may be utilized, and for data set D_(2,15),projection operators p₅₃, p₅₄, p₆₁, and p₆₂ may be utilized. Further,with respect to the structure, FIG. 10E shows the tree structure 1040 ofthe sub-areas of D that were analyzed during the process.

In view of foregoing and now referring to FIG. 11, an example method1101 for large volume holographic imaging is provided. The examplemethod 1101 may be performed, for example, by the image processor 130via the processing circuitry 505. In this regard, the example method1101 may comprise, at 1100, decomposing a target volume into a pluralityof sub-volumes, and, at 1110, determining projection operators forpoints within each sub-volume based on an architecture of a sensor arrayand a spatial relationship of the sensor array to the target volume.Additionally, the example method 1101 may also comprise determining apoint aggregation operator for each sub-volume at 1120. In this regard,the point aggregation operator may be an aggregation of the projectionoperators associated with a respective sub-volume. Also, at 1130, theexample method 1101 may also comprise receiving holographic fieldmeasurement data set captured for the target volume via the sensorarray, and, at 1140, generating a sub-volume interest value for eachsub-volume by applying the holographic field measurement data set toeach point aggregation operator. Further, at 1150, the example method1101 may comprise determining a sub-volume with a highest sub-volumeinterest value, and, at 1160, determining respective lower-tiersub-volume interest values for lower-tier sub-volumes of the sub-volumewith the highest sub-volume interest value. In this regard, thelower-tier sub-volumes may be defined by decomposing the sub-volume withthe highest sub-volume interest value. Additionally, at 1170, theexample method 1101 may comprise generating an image of the targetvolume based on the lower-tier sub-volume interest values.

Additionally, according to some example embodiments, the example method1101 may further comprise ordering the sub-volumes in a stack. In thisregard, the sub-volume with the highest sub-volume interest value may befirst in the stack and a sub-volume with a lowest sub-volume interestvalue may be last in the stack. Additionally or alternatively, accordingto some example embodiments, the example method 1101 may furthercomprise discarding from further analysis sub-volumes associated withsub-volume interest values that do not satisfy an elimination threshold.Additionally or alternatively, according to some example embodiments,generating the image of the target volume based on the lower-tiersub-volume interest values may comprise rendering a voxel representationof the lower-tier sub-volumes based on the respective lower-tiersub-volume interest values. Additionally or alternatively, according tosome example embodiments, determining the point aggregation operatorsmay comprise, for each sub-volume, averaging or summing the projectionoperators for the points within the sub-volume. Additionally oralternatively, according to some example embodiments, the example method1101 may further comprise determining a lower-tier point aggregationoperator for at least one lower-tier sub-volume prior to receiving theholographic field measurement data set. Additionally or alternatively,according to some example embodiments, the example method 1101 mayfurther comprise generating an operator tree structure for the pointaggregation operators in association with sub-volumes and the lower-tierpoint aggregation operators in association with the lower-tiersub-volumes. Additionally or alternatively, according to some exampleembodiments, the target volume and the sub-volumes may be decomposed viaan octree decomposition. Additionally or alternatively, according tosome example embodiments, the example method 1101 may further comprisegenerating a security alert for security personnel based on thegenerated image.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Moreover, although the foregoing descriptions and the associateddrawings describe exemplary embodiments in the context of certainexemplary combinations of elements and/or functions, it should beappreciated that different combinations of elements and/or functions maybe provided by alternative embodiments without departing from the scopeof the appended claims. In this regard, for example, differentcombinations of elements and/or functions than those explicitlydescribed above are also contemplated as may be set forth in some of theappended claims. In cases where advantages, benefits or solutions toproblems are described herein, it should be appreciated that suchadvantages, benefits and/or solutions may be applicable to some exampleembodiments, but not necessarily all example embodiments. Thus, anyadvantages, benefits or solutions described herein should not be thoughtof as being critical, required or essential to all embodiments or tothat which is claimed herein. Although specific terms are employedherein, they are used in a generic and descriptive sense only and notfor purposes of limitation.

That which is claimed:
 1. A method for large volume holographic imaging,the method comprising: decomposing, by processing circuitry, a targetvolume into a plurality of sub-volumes; determining, by the processingcircuitry, projection operators for points within each sub-volume basedon an architecture of a sensor array and a spatial relationship of thesensor array to the target volume; determining, by the processingcircuitry, a point aggregation operator for each sub-volume, the pointaggregation operator being an aggregation of the projection operatorsassociated with a respective sub-volume; receiving, by the processingcircuitry, a holographic field measurement data set captured for thetarget volume via the sensor array; generating, by the processingcircuitry, a sub-volume interest value for each sub-volume by applyingthe holographic field measurement data set to each point aggregationoperator; determining, by the processing circuitry, a sub-volume with ahighest sub-volume interest value; determining, by the processingcircuitry, respective lower-tier sub-volume interest values forlower-tier sub-volumes of the sub-volume with the highest sub-volumeinterest value, the lower-tier sub-volumes being defined by decomposingthe sub-volume with the highest sub-volume interest value; andgenerating, by the processing circuitry, an image of the target volumebased on the lower-tier sub-volume interest values.
 2. The method ofclaim 1, further comprising ordering the sub-volumes in a stack, whereinthe sub-volume with the highest sub-volume interest value is first inthe stack and a sub-volume with a lowest sub-volume interest value islast in the stack.
 3. The method of claim 1, further comprisingdiscarding from further analysis sub-volumes associated with sub-volumeinterest values that do not satisfy an elimination threshold.
 4. Themethod of claim 1, wherein generating the image of the target volumebased on the lower-tier sub-volume interest values comprises rendering avoxel representation of the lower-tier sub-volumes based on therespective lower-tier sub-volume interest values.
 5. The method of claim1, wherein determining the point aggregation operators comprises, foreach sub-volume, averaging or summing the projection operators for thepoints within the sub-volume.
 6. The method of claim 1, furthercomprising determining a lower-tier point aggregation operator for atleast one lower-tier sub-volume prior to receiving the holographic fieldmeasurement data set.
 7. The method of claim 6, further comprisinggenerating an operator tree structure for the point aggregationoperators in association with sub-volumes and the lower-tier pointaggregation operators in association with the lower-tier sub-volumes. 8.The method of claim 7, wherein the target volume and the sub-volumes aredecomposed via an octree decomposition.
 9. The method of claim 1,further comprising generating a security alert for security personnelbased on the generated image.
 10. An apparatus for large volumeholographic imaging, the apparatus comprising processing circuitryconfigured to: decompose a target volume into a plurality ofsub-volumes; determine projection operators for points within eachsub-volume based on an architecture of a sensor array and a spatialrelationship of the sensor array to the target volume; determine a pointaggregation operator for each sub-volume, each point aggregationoperator being an aggregation of the projection operators associatedwith a respective sub-volume; receive holographic field measurement dataset captured for the target volume via the sensor array; generate asub-volume interest value for each sub-volume by applying theholographic field measurement data set to each point aggregationoperator; determine a sub-volume with a highest sub-volume interestvalue; determine respective lower-tier sub-volume interest values forlower-tier sub-volumes of the sub-volume with the highest sub-volumeinterest value, the lower-tier sub-volumes being defined by decomposingthe sub-volume with the highest sub-volume interest value; and generatean image of the target volume based on the lower-tier sub-volumeinterest values.
 11. The apparatus of claim 10, wherein the processingcircuitry is further configured to order the sub-volumes in a stack,wherein the sub-volume with the highest sub-volume interest value isordered first in the stack and a sub-volume with a lowest sub-volumeinterest value is ordered last in the stack.
 12. The apparatus of claim10, wherein the processing circuitry is further configured to discardfrom further analysis sub-volumes associated with sub-volume interestvalues that do not satisfy an elimination threshold.
 13. The apparatusof claim 10, wherein the processing circuitry configured to generate theimage of the target volume based on the lower-tier sub-volume interestvalues includes being configured to render a voxel representation of thelower-tier sub-volumes based on the respective lower-tier sub-volumeinterest values.
 14. The apparatus of claim 10, wherein the processingcircuitry configured to determine the point aggregation operatorsincludes being configured to, for each sub-volume, average or sum theprojection operators for the points within the sub-volume.
 15. Theapparatus of claim 10, wherein the processing circuitry is furtherconfigured to determine a lower-tier point aggregation operator for atleast one lower-tier sub-volume prior to receiving the holographic fieldmeasurement data set.
 16. The apparatus of claim 15, wherein theprocessing circuitry is further configured to generate an operator treestructure for the point aggregation operators in association withsub-volumes and the lower-tier point aggregation operators inassociation with the lower-tier sub-volumes.
 17. The apparatus of claim16, wherein the processing circuitry is further configured to decomposethe target volume and the sub-volumes via an octree decomposition.
 18. Aholography imaging system comprising: a holography sensor comprising asensor array, the holography sensor configured to capture holographicfield measurement data set of a target volume; and processing circuitryconfigured to: decompose the target volume into a plurality ofsub-volumes; determine projection operators for points within eachsub-volume based on an architecture of the sensor array and a spatialrelationship of the sensor array to the target volume; determine a pointaggregation operator for each sub-volume, each point aggregationoperator being an aggregation of the projection operators associatedwith a respective sub-volume; receive the holographic field measurementdata set captured for the target volume via the sensor array; generate asub-volume interest value for each sub-volume by applying theholographic field measurement data set to each point aggregationoperator; determine a sub-volume with a highest sub-volume interestvalue; determine respective lower-tier sub-volume interest values forlower-tier sub-volumes of the sub-volume with the highest sub-volumeinterest value, the lower-tier sub-volumes being defined by decomposingthe sub-volume with the highest sub-volume interest value; and generatean image of the target volume based on the lower-tier sub-volumeinterest values.
 19. The holography imaging system of claim 18, whereinthe processing circuitry is further configured to generate an operatortree structure for the point aggregation operators in association withsub-volumes and lower-tier point aggregation operators in associationwith the lower-tier sub-volumes.
 20. The holography imaging system ofclaim 19, wherein the processing circuitry is further configured todecompose the target volume and the sub-volumes via an octreedecomposition.